OBD-II trouble code
P0356: Ignition Coil 'F' Primary/Secondary Circuit
The sixth coil in the manufacturer's mapping, and the last one a six-cylinder engine can report. On a transverse V6 that coil is usually against the bulkhead — which makes this a labour bill more than a parts bill, and changes the order you should test in.
Quick facts
- System
- Powertrain
- Category
- Ignition
- Severity
- Medium severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $60 – $900
- DIY difficulty
- Advanced DIY
What does P0356 mean?
Six is a boundary. An engine that can report coil 'F' has at least six independently driven ignition outputs, and for the overwhelming majority of vehicles that means a six-cylinder engine — a V6, a straight six, or on rarer machinery a flat six. On all of those, 'F' is the last letter available, which makes this the code at the top of the range for the second most common engine layout on the road after the inline four.
The consequence that dominates everything else is access, and specifically the transverse V6. Mount a V engine sideways in a front-wheel-drive car and one bank of cylinders ends up facing forward with clear space above it, while the other bank ends up pressed against the bulkhead with the upper intake manifold, the throttle body, wiring and hoses stacked on top of it. On a great many of those engines the rear bank coils cannot be reached at all until the upper intake plenum comes off. That is a one-to-two hour job on its own, and it usually needs new intake gaskets to put back together properly.
That single fact should reorder your diagnosis. On an accessible coil, the fastest test in the book is to swap it with a neighbour and see whether the fault moves. On a rear-bank transverse V6 that test costs the same disassembly as the repair itself, so performing it as a first move throws away the cheapest thing you have. Do the tests that can be done from outside first: read every stored code and let a cylinder-specific misfire identify the cylinder; check the coil supply fuse and relay; look at whether other coils on the same bank are also complaining, which would point at the branch rather than at one unit. Only when those come back clean is it worth committing to opening the intake.
And once you are committed, the economics invert. The labour is already spent, the plenum is already off, and the marginal cost of replacing all three rear-bank coils and their spark plugs while you are in there is small — parts only, on a job whose cost was mostly labour. Coming back in eight months to do the same disassembly for the coil next to the one you just changed is the outcome worth spending a little to avoid. This is the one place in the ignition coil family where replacing more than the failed part is straightforwardly good value rather than an upsell.
Straight sixes and flat sixes escape most of this. On a longitudinal straight six every coil sits in a row along the top of one head with nothing over it, and 'F' is simply the coil closest to the bulkhead — accessible, cheap to reach, and no different in labour from the first one. The same code number on those engines is a much smaller bill, which is why the engine layout is worth establishing before anyone quotes you a figure.
Common causes
- Failed ignition coil on the sixth output, most often a rear-bank cylinder on a transverse V6
- Coil connector disturbed or left unlatched during previous intake plenum work
- Heat-hardened coil boot and potting compound on a rear-bank coil trapped under the intake with poor airflow
- Oil in the spark plug well from a leaking valve cover gasket on the rear head, which is the harder cover to inspect
- Chafed rear-bank coil harness where it passes beneath the upper intake manifold
- Worn spark plug that has gone past its service interval because the rear bank was skipped at the last plug change
- Failed coil driver inside the engine control module
- Corroded ground or supply connection shared by the rear bank coils
Symptoms
- Check engine light on, sometimes flashing during an active misfire
- Misfire on one rear-bank cylinder, most obvious at idle and under load
- Rough idle with a vibration felt through the floor rather than the wheel
- Loss of power and hesitation, particularly when pulling away or climbing
- Fault appearing months after intake manifold, plenum or coolant work
- Poor fuel economy and a smell of unburnt fuel
Diagnostic steps
- 1.Establish the engine layout first. A rear-bank coil on a transverse V6 is a fundamentally different job from the same coil on a straight six, and the quote should reflect that before any work starts.
- 2.Read every stored code and use a cylinder-specific misfire to identify the cylinder, rather than assuming the letter maps to a position.
- 3.Do every test that can be done without disassembly before opening anything: coil supply fuse, relay, battery voltage, and whether more than one coil on the same bank is reporting.
- 4.Check whether the other rear-bank coils are also flagging. Several coils on one bank points at a shared supply, ground or harness branch rather than at one failed unit.
- 5.Resist the swap test as a first move on a rear-bank transverse V6. It costs the same disassembly as the repair, so spend it only once the cheaper checks are exhausted.
- 6.Once the plenum is off, inspect all the rear-bank plug wells for oil before fitting anything. A leaking rear valve cover gasket is much harder to spot from outside and will ruin a new coil.
- 7.Compare primary resistance across all six coils while you have access, so a marginal one is caught now rather than on a second teardown.
- 8.Replace the rear-bank spark plugs at the same time if they are anywhere near their interval. The labour is already paid for and the parts are cheap by comparison.
Repair cost
$60 – $900
The coil itself is typically $50 to $220. What moves the number is access. On a straight six or a longitudinal V6 with clear coils, expect $90 to $300 all in. On a transverse V6 where the upper intake plenum must come off to reach the rear bank, labour is commonly one to two and a half hours plus intake gaskets, taking the total to $350 to $900. Because the labour dominates, replacing all three rear-bank coils and plugs during the same teardown adds parts cost only and is usually the better value.
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DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.